Current lithium-ion batteries, or cells, use a solid reductant as the anode and a solid oxidant as the cathode. Solid-state, high energy-density batteries use metallic lithium as the anode. Lithium metal is a preferred anode material as a result of its superior thermodynamic and kinetic properties. In addition, lithium is a good conductor of electricity and heat. Furthermore, lithium's malleability and ductility make it an excellent metal with which to work. However, lithium has well-known drawbacks when used as an anode material. For example, lithium is very reactive and often creates inflammability concerns.
A battery consists of three basic parts—two electrodes (a cathode and anode) separated by an electrolyte. Lithium ion batteries use host materials for the electrodes (for example, carbon as the anode and lithium cobalt oxide as the cathode) to avoid using metallic lithium, thereby improving safety. Electrochemical reactions at the electrodes produce an electric current that powers an external circuit. When the battery is discharged, the anode supplies Li+ ions to the Li+ ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li+ ions are inserted reversibly from the electrolyte as a guest species and are charge-compensated by electrons from the external circuit. During charge and discharge of lithium ion rechargeable batteries, lithium ions are shuttled between the cathode and anode host materials in a “rocking horse” fashion. Primary batteries or cells are those in which the chemical reaction supplying the electrons is not reversible with respect to the closed universe of the battery. A secondary battery, or cell, utilizes a reaction which can be reversed when current is applied to the battery, thus “recharging” the battery. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, the removal of electrons from the cathode by an external field releases Li+ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li+ ions back into the anode to restore it to its original composition.
A polar aprotic solvent is typically used as the liquid electrolyte solvent in lithium batteries. Aprotic solvents are used due to the absence of labile hydrogen atoms, which would react with lithium to release hydrogen. Polar solvents are those having a strong dipole moment in the molecule. They are used both because they have substantial solvation energies for the electrolyte salt which results in better salt dissolution of the salt, and because they have a higher dielectric constant for the solvent, i.e., better ionic dissociation.
Common solvents that have been used in lithium batteries either in pure form or in solvent mixtures include but are not limited to propylene carbonate (“PC”), ethylene carbonate (“EC”), diethyl carbonate (“DEC”), 1,2-dimethoxyethane, and methylformate. These solvents provide the necessary conductivity in the lithium-ion cell.
There are at least two detrimental effects that stem from the reaction of the lithium with the electrolyte: (1) the exothermic liberation of heat and (2) the formation of a passivating film on the anode's surface. The exothermic release of energy is a problem because an explosive release of energy and reactive materials can result, thus creating a hazard for both the operator and the device that is being powered by the battery. This release of heat often occurs when primary batteries are subjected to temperatures above the recommended levels or when secondary cells are subjected to unusual or severe conditions of recharging. Primary lithium cells using, for example, a lithium thionylchloride system, have been known to undergo high exothermic reactions when subjected to temperatures above or below the recommended temperatures. In the case of secondary cells, subjecting the cells to unusual or severe recharging conditions and deposition of lithium in a highly porous film on the anode have led to similar disastrous results.
Furthermore and as previously mentioned, a dendritic layer will form in the cell. Due to the intrinsic reactivity of lithium toward the electrolyte, the lithium in secondary cells will deposit to form a dendritic layer, which enhances the reactivity of lithium. This formation of passive films on lithium has been shown to be one reason for the loss of capacity of lithium cells on repeated cycling. The film can isolate the anode from the electrolyte, thereby providing a high impedance path and a degradation in cell performance. In addition, lithium metal tends to “deposit out” on the surface of the film rather than on the lithium anode; the deposited lithium is electrically isolated from the anode and is unavailable for later discharges.
The problem of lithium reactivity toward the electrolyte has been addressed in various ways. One approach is to use a carbon intercalation compound such as LiC6 or LiC12 with either a liquid or polymeric electrolyte. One disadvantage of this approach is the loss of capacity density.
Interest in phosphate compounds as insertion materials for lithium-ion batteries has led to a large number of studies. One example of these phosphates has the formula of NaZr2(PO4)3 which is the basic member of a large family called Nasicon (Na super-ionic conductor). The first intercalation reactions were found in LiTi2(PO4)3 and NaZr2(PO4)3 having a similar structural arrangement.
Although these compounds have a high capacity of lithium intercalation, the kinetics of the electrochemical reaction are very slow, which is caused in large part by their poor electronic conductivity. The inherent drawbacks of lithium metal resulted in a concerted effort to formulate alternative anodes, cathodes, and or electrolytes that could produce a battery having the improved performance profile of a lithium-ion batter without the environmental, economic, and safety concerns. Much of the cathode research has focused on finding a cheaper substitute for the traditional cathodes such as LiCo2O2. In addition, there is a need for cathode materials that have a better electronic conductivity allowing for faster kinetics in the electrochemical reaction. Improved liquid organic electrolytes with higher conductivities than these solvents are needed to meet the power requirements for many consumer applications, especially at low temperatures.